Molded polymer foams such as polystyrene are most commonly made commercially either by extruding a molten polymer that contains a blowing agent, or by expanding and molding polymer particles that contain a blowing agent. When polymer particles are used, the particles are typically impregnated with a hydrocarbon blowing agent such as pentane, then heated with saturated steam to give "pre-expanded" polymer particles, i.e., larger particles of lower density. The pre-expanded particles, commonly called "prepuff," must be aged for at least about two hours to allow the particles to mature. During the aging process, air permeates the particles, and the internal pressure of the particles gradually approaches atmospheric pressure. The pre-expanded particles are commonly stored in large vented bags during the aging process. A substantial amount of storage space is required and handling of the bags is cumbersome. In addition, the need to age the particles wastes valuable time. A desirable process would eliminate the need for storage of pre-expanded beads, and would allow beads to be molded immediately following pre-expansion.
U.S. Pat. No. 3,666,850 teaches a method and apparatus for heating expandable polymer particles in a gun, and using a stream of hot air to soften and convey the particles to a zone in which the particles can expand and fuse to produce a foamed mass around an object to be packaged. The process permits encapsulation of electronic parts in a moisture-free protective foam.
U.S. Pat. No. 3,278,658 teaches a process for making a foamed thermoplastic article by heating pre-expanded polymer particles with hot air to soften the particles, transferring the particles to the desired molding zone, and subjecting the particles to a vacuum so that they expand to fill the space and fuse together.
U.S. Pat. No. 5,049,328 teaches a method of purifying, impregnating, and molding polymer particles with carbon dioxide. Low molecular weight impurities are advantageously removed from the polymer particles by extraction, especially with supercritical carbon dioxide. Because carbon dioxide rapidly diffuses from the polymer particles, CO.sub.2 -impregnated particles cannot be stored except under pressure for any long period.
Still needed in the art is a process for making polymer foams from expandable particles that eliminates the need to store and age an intermediate pre-expanded product. Preferably, the process could use expandable particles that have been purified and/or impregnated with carbon dioxide to eliminate the need for flammable hydrocarbons or ozone-depleting chlorofluorocarbons as blowing agents.